Move the harness systemd unit rationale (role-driven unit name,
manager-only forge defaults, PATH wrapper-dir trick, env vars,
standalone-eval fallbacks, RuntimeDirectory + User= reasoning)
from `nix/templates/harness-base.nix` to a new
`docs/agent-hierarchy.md::Harness systemd unit shape (per-role)`
section.
In-code comments trim to short purpose statements + pointers; the
PATH /bin auto-append behaviour is already documented in
docs/gotchas.md, so the harness file just cross-refs both.
`description = ''…''` blocks (operator-facing options docs)
preserved per iris #718.
`nix flake check` clean; `nix fmt` clean (after formatter pass).
